Unravelling of physiologicai functions of retinoid using a dominant-negative receptor

Although pharmacologicai doses of retinoic acid (RA) have been shown to have a wide variety of actions in vivo, a definitive assignment of the physiological functions has not yet been made due to the lack of any effective methodology. We have recently generated a dominant-negative retinoic acid receptor (RAR) by a single amino acid substitution The mutated RAR efficiently inhibited the endogenous activities of RARs. Thus, targeted expression of the mutated receptor can be expected to reveal RA functions during organogenesis by blocking RA-signaling in the tissues concerned. To address this possibility, we expressed the dominant-negative RAR exclusively in the epidermis, a potential target organ of RA. The resultant transgenic mice exhibited dramatic suppression of epidermal development. Notably, the affected skin lacks keratinization and wrinkles, demonstrating the absolute requirement of RA in normal skin development. This method is theoretically applicable to every organ, thus opening the way to define the physiological roles of RA during embryogenesis as well as in adults. Furthermore, the application of dominant-negative molecules has been proven to overcome the limitations of gene-targeting, in cases in which the genes consist of multiple members like RARs, or the gene will lead to embryonic death without the gene product.